Remove the HangWatcherBlockingThreadTest fixture

After the cleanup in previous CLs, the only remaining purpose of the
fixture was to hold the BlockedThread instance. We can move that
instance to the test themselves and remove the fixture.

Hiding the thread in the test fixture wasn't ideal in the first place.
The thread is configured with a certain hang deadline time and the test
body fast forward time to cause the hang. Thus, the deadline is in a way
a test input. In general terms, tests should never explicitly rely on
input values defined in the fixture setup.

The use of the kTimeout and kHangTime constants was dropped because
these adds indirection and and make the test harder to understand and
validate. By seeing the constants in the test body directly, it makes it
clear to the reader that the time is fast-forwarded passed the timeout.

Bug: 413314866
Change-Id: Ib4c54c5f38b1d675b4fee67f99593be039673fbc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6578892
Commit-Queue: Jean-Philippe Gravel <jpgravel@chromium.org>
Reviewed-by: Olivier Li <olivierli@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1466475}